A few days ago I recall finding a visual calendar of my sign-in history on Math Stack Exchange. It looked like an ordinary monthly calendar, where the days were white if I hadn't signed in, and green if I had. (I found it when I wondering how close I've been in the past to getting the "fanatic" badge.)

However, I can no longer find this page/pop-up.

I thought it might have been a feature of Stack Exchange as a whole, but I was unable to find an answer on Meta Stack Exchange.

$\begingroup$A brief answer is that you should go to your profile and click on the number of consecutive days. (Remember that you'll find this in the profile tab, not in the activity tab.)$\endgroup$
– Martin SleziakOct 28 '17 at 7:26

$\begingroup$Since you explicitly mentioned that this was interesting for you in part in connection with a badge, I'll add also link to this question: Progress towards badges. And since that answer was posted, also the new feature called badge tracking was introduced.$\endgroup$
– Martin SleziakOct 28 '17 at 7:40

(Originally I considered that this question could be closed as a duplicate of the first one in this list. But maybe they are slightly different after all - the other one asks also whether it is possible to see this information for other users, not just your own visits. That's why I decided to convert my comments to an answer.)

When you click your own profile, the default tab is the activity tab. You should go to the "Profile" tab, where you can see the "visited $x$ days, $y$ consecutive" text. Click this text to show the calendar.

$\begingroup$Sure. Those two just had a head-start. I believe to recall there is somebody on tex.se too with a comparable record. But I cannot locate the meta.se post where it came up. something about a display bug maybe.$\endgroup$
– quid♦Oct 29 '17 at 23:31

$\begingroup$Well, I'm sure there are others like me who check the website every day, and have been doing that for a long enough period. It's not hard, just requires basic dedication... :)$\endgroup$
– Asaf Karagila♦Oct 29 '17 at 23:32